Currently, there are two types of USB ports, 100 mA type and 500 mA type. When using USB port for a battery charger, if the USB port is a 100 mA type but 500 mA is sinking to a charged battery, the voltage on the USB port would be loading to fail. Contrarily, if the USB port is a 500 mA type but only 100 mA is sinking to the charged battery, the charge time will be long and the charge efficiency is low. However, it is difficult for the end users and system designers to know or predict which USB type will be used.
On the other hand, when using a multi-ports USB for battery charging, the charge current setting and charge efficiency is a trade-off. If the charge current setting is low, the charge time will be long so that the charge efficiency is low. If the charge current setting is high in order for short charge time and high charge efficiency, the power supply will be easily overloading when other device also uses other port of the USB at the same time.
Therefore, it is desired an automatic switch method and apparatus for a USB.